If we can have Eponine's who are a different colour to their parents, and Fantine's who are different again to their child, there's no problem with non hispanic Marias. All you need are people who look like gang members. and in any, conceivably casted, gang on a UK stage show, if you worry about race at all , one hispanic is going to look out of place if they are the only one there.
There's a bizarre logic applied here too. At one level how can it be fine to have brits playing Americans, or all sorts of Europeans, but not hispanics? On another level, Puerto Ricans are Puerto Ricans and proud of it. They are not acme generic Hispanics - as if anyone from Portugal or Brazil or Mexico or New Mexico would do. I am still trying to work out the logic of employing Philppinos as Vietnamese for Miss Saigon - it must look odd to the Vietnamese who have territorial disputes with the Philippines, and when the Philipinnes deployed troops on the US side in the war. I don't think we need to scour the UK for a Puerto Rican who can actually sing Maria too.